1. Field of the Invention
The present invention relates to fine wire bonding employing ultrasonic energy applied to the fine wire by a bonding tool held and controlled by an automatic wire bonder. More particularly, the present invention relates to a system and a process of multi-frequency wire bonding wherein a plurality of different resonant frequencies are applied to the same transducer during a single wire bond operation.
2. Description of the Prior Art
Heretofore, ultrasonic transducers used for fine wire bonding were designed and manufactured for use at a single high efficiency resonance frequency. It was generally known that such transducers had one efficient resonant frequency, however, it was not known or appreciated that such transducers could be made which had two distinct resonance frequencies until disclosed in copending U.S. Ser. No. 08/349,251 filed Dec. 5, 1994 for a Multi Resonance Unibody Ultrasonic Transducer by Ali R. Safabakhah and assigned to the same assignee as the present invention.
Heretofore, it was generally known that fine wire bonds made at 60 Khz form in a different manner and display different shear strength characteristics from fine wire bonds made at 120 Khz. There is little or no available data defining the intermetallic structure and shear bond strength for wire bonds made at frequencies that dramatically differ from these frequencies and their closely associated frequencies. Lee Levine summarized the state of art theories concerning wire bonds made at these frequencies in his article "The Ultrasonic Wedge Bonding Mechanism: Two Theories Converge" presented at the International Symposium on Microelectronics (ISHM) Proceedings dated Oct. 24-26 , 1995 at pages 242 to 246.
Summarized, this article disclosed that wire bonds made at high frequencies (e.g. 120 Khz) cause the material at the interface of the wire bond to have high rates of strain hardening accompanied by slight deformation in the initial stages of forming a bond. All things being equal wire bonds made at low frequencies (e.g. 60 Khz) have lesser rates of strain hardening accompanied by greater squash and bond area resulting in stronger shear strength wire bonds.
It would be desirable to provide a system and a method for making fine wire bonds employing two or more resonance frequencies so as to take advantage of all the desirable characteristics which can be achieved using either high or low resonance frequencies. Multi-resonance frequency bonding transducers having two or more efficient resonance frequencies will now permit the incorporation of multi-resonance frequency methods and systems into a new automatic wire bonders as well as permitting retrofitting such systems into existing automatic wire bonders.